Something to watch out for is the orientation of your buildings. It's very easy when using a square brush for all of your buildings to be strictly oriented to the compass, but likely they wouldn't be. If you want to just add some random rotation, open your Brushes palette (F5) and under Brush Dynamics, turn up the Angle Jitter.

If you want the buildings oriented to the street, I think you have to put each building on its own layer and rotate them by hand before merging down. There may be a way to get the rotation to follow a path, but I haven't divined how to do it yet.